1. Field of the Invention
This invention relates to wind turbines housed in a nacelle atop a tall tower, and more particularly to an apparatus and method for raising or lowering a rotor blade of a wind power installation for connection to a hub or disconnection from the hub.
2. Description of the Prior Art
The average wind turbine size is now approximately 1.5 MW with rotor diameters ranging from 70 to 85 m. This class of turbines has on average a nacelle weight of 50 tons (generators, gear box and nacelle housing) and a rotor weight of approximately 75 tons. To improve the economics of wind turbines, manufacturers have been designing taller towers to take advantage of greater wind energy at higher levels above ground. Manufacturers are increasingly relying on 80 and 100 m tower designs, rather than the shorter towers used previously. The cost of lowering and lifting rotor blades to the top of tall land based towers for servicing can be prohibitive, mainly due to the reach of conventional cranes, which require much higher crane capacity than the actual weight being lifted. This is due to the bending moment resulting from the reach of conventional cranes. Currently wind turbines require that a crane or special apparatus to be brought to the wind turbine site for accessing the blades through removal of each blade, in order to remove the blades from the rotor hub to which it is attached and lower them down to the ground for repair or replacement. This adds a substantial cost for repair and blade replacement activity leading to an increase in the cost of energy.
Cranes are also used for mounting rotor blades to a wind power installation. A crane raises the rotor blade from the base of the wind power installation and takes it to the rotor blade connection at the hub of the wind power installation. Bolts through a flange of the rotor blade into a mating part of the hub make the connection of the blade to the hub. Nuts are screwed onto the bolts to secure the blade to the hub.
US Patent Application Publication US 2009/0058096 (Wobben) discloses a method of mounting or dismantling a rotor blade of a wind power installation without using a crane. A cable is stretched between a part in the outside surface of the hub of the wind power installation and the bottom region of the wind power installation. The rotor blade is moved along the cable upwardly upon mounting or downwardly upon dismounting the blade.
German Patent DE 20 2004 016460 U1 discloses a block-and-tackle mechanism for raising and lowering sections of a rotor blade. The mechanism is attached to the outside surface of the hub and the outside surface of the rotor blade.
PCT/JP2008/059474 published Dec. 24, 2008 of Tomohiro Umajiri, et al. discloses a method of mounting a rotor blade, which allows the blade to be mounted and removed without using multiple construction machines. The suspension device has a pulley inside the hub, a holding section inside the blade, and a winch. The pulley leads a suspension wire from the rotor hub, on which the blade is to be mounted, to the holding section fixed at the root end and inside the blade. The holding section holds the mounting end side of the blade such that a line connecting the center of gravity of the blade and a connection section to which the suspension wire is connected intersects with the longitudinal axis of the blade. The winch is placed on the ground and raises and lowers the holding section/blade by pulling in and paying out the suspension rope.
The above-described publication has the disadvantage that it does not provide a way of accurately guiding the blade into engagement with the hub connection flange. It also adds weight to each blade by use of the holding section within the blade, which cannot be removed from the blades once they are mounted.
It is desirable to provide an apparatus and method, which facilitates the repair and replacement of wind turbine blades and lowers the costs of such repair and replacement.